Plug-in connectors are known from DE 199 35 246 A1 and DE 102 21 971 A1, and contain a base body, an annular sealing element, a retaining element, and a sleeve that can be connected to the base body. A tube end that can be inserted into an annular gap between the base body and the sleeve can be fixed in the plug-in connector by means of the annular retaining element, which is also called a clamping ring, whereby the annular sealing element arranged on the base body is sealingly positioned against the interior surface of the inserted tube end. The retaining element is divided largely in the direction of the longitudinal axis of the plug-in connector by a gap and contains at least one claw that engages radially in the exterior surface of the tube end for fixing the latter in the plug-in connector. There can be difficulties in terms of functional fixation of the tube end in the plug-in connector, in particular as a result of production tolerances for the sleeve and for the retaining element that is divided in the longitudinal direction and furthermore for the tubes. Moreover, problems can occur due to the claw or claws of the retaining element penetrating too deep into the exterior surface of the tube, whereby disadvantageous damage to the intermediate metal layer can occur when the tube is embodied as a plastic/metal compound tube with an intermediate metal layer that is in particular made of aluminum.
Starting at this point, the object of the invention is to further develop the plug-in connector of the type cited in the foregoing such that high functionality of the connecting and fixing of the tube end in the plug-in coupling is attained with low production and assembly complexity. The plug-in connector should facilitate functionally correct handling during assembly, and should preclude the risk of assembly errors. The plug-in connector should also be able to be used for tubes made of different materials, such as plastic or metal or a composite thereof in the form of compound tubes, and for tubes made of comparatively soft materials or plastics, such as in particular polyethylene. It should be possible to produce the plug-in connector with no problem, and the plug-in connector should ensure a lasting and secure connection of tube to plug-in connector for many years. Furthermore, the plug-in connector should be able to be used in the field of sanitation, in particular for hot water and cold water installation. Above all, the plug-in connector should be able to reliably withstand changes in the material, in particular with regard to the dimensions or rigidity of the tube, that are caused by changes in temperature in the flowing medium, whereby functional sealing and connection should be assured for a service life that is decades-long.